1. Field of the Invention
The present invention concerns a plasma panel with its electrodes arranged in a novel way so as to make it possible, notably, to increase the speed of obtaining the images displayed by this panel.
2. Description of the Prior Art
Plasma panels are flat panel or flat screen display devices that are now well known. They enable the display of alphanumerical, graphic or other images, in color or otherwise. Generally, plasma panels comprise two insulating plates bounding a volume occupied by a gas (generally a neon-based mixture). These plates support conductive electrodes, placed in columns and called column electrodes, and electrodes placed in rows, called row electrodes. These column and row electrodes intersect or cross one another so as to define a matrix of cells each forming a picture element or pixel. The working principle is the selective generation, at the crossing of row electrodes and column electrodes, namely at selected pixels, of electrical discharges in the gas. Data is displayed through an emission of light which accompanies theses discharges.
Certain plasma panels work in DC mode, but it is most commonly preferred to use so-called "AC" type panels, the working of which is based on an excitation of the electrodes in AC mode. In this case, the electrodes are coated with a layer of dielectric material and are no longer in direct contact either with the gas or with the discharge. One of the advantages of this type of plasma panel, called an "AC" plasma panel, is that it has a memory effect, enabling the useful information to be presented solely to the pixels for which it is desired to change the state (lit up or extinguished). At the other picture elements or pixels, the state of these pixels is simply sustained by repetition of alternating electrical discharges, called sustaining discharges, obtained solely for the pixels that are in the lit up state, i.e. written.
Under these conditions, the control of the pixels may consist in a point-by-point, i.e. pixel-by-pixel addressing operation, so that the duration of the addressing time, which limits the data refreshing rate, is not generally a problem.
It has to be noted that certain so-called AC type plasma panels use only two electrodes to define a pixel: one column electrode intersected with a row electrode. The working of a plasma panel of this type is known, notably from a French patent No. 78 04 893, filed on behalf of THOMSON-CSF and published under No. 2 417 848. This method also describes a method for the control of a panel such as this.
There are also known plasma panels, called "coplanar sustaining plasma panels", in which three or more electrodes are used to define a pixel. In this case, most often, each pixel of the matrix is formed by three electrodes, more precisely at the intersection between a column electrode and two parallel sustaining electrodes forming a pair of sustaining electrodes. With this type of screen, it is known that the sustaining of the discharges, namely the repetition of the above-mentioned alternating electrical discharges, is done between the two sustaining electrodes of one and the same pair, and that the addressing is done by the generation of discharges between two intersecting electrodes. In this case, the column electrode has a solely addressing function, and among the two electrodes of one and the same pair of electrodes, one electrode has a solely sustaining function while the other electrode fulfils a sustaining function and an addressing function.
A plasma panel of the AC coplanar sustaining type, with three electrodes per pixel, is known notably from the European patent document EP-A-0 135 382, which also describes a method for the control of this panel. At each pixel, the sustaining electrodes may have a protuberance or projecting surface: in one and the same pair of sustaining electrodes, the projecting surfaces of an electrode are pointed towards those of the other electrode, and the sustaining discharges occur between these projecting surfaces.
Another structure of the coplanar sustaining AC type is described, with its control system, in an article by G.W. DICK in PROCEEDINGS OF THE SID, vol 27/3, 1986, pages 183-187. It must be noted that, in the structure described in this document, the sustaining electrodes have a constant width, that is, they have no facing, projecting surfaces in a pair of sustaining electrodes, to define the sustaining discharge zone. By contrast, they have barriers made of an insulating material. These barriers serve to confine sustaining discharges in the zone of intersection with the column electrode.
In all these types of plasma panel, the column electrodes are individualized so that it is possible to select only one of them, i.e. they are each connected to a particular output of a control and addressing device. This is also so for the row electrodes in the case where a pixel is defined at the intersection of a column electrode and a single row electrode (for DC as well as AC type plasma panels). As regards the coplanar sustaining plasma panels, those of the sustaining electrodes that fulfil the function of sustaining the discharges and the addressing function (addressing-sustaining electrodes) are also all individualized.
Irrespectively of the type of plasma panel, the data refreshing rate is not generally a problem when the control method used is of the point-by-point addressing type. However, there are applications where it is desired to have the ability for addressing as rapidly as possible. These are, in particular, plasma panels which are required to have compatibility with standard video signals and for which, in particular, it is desired to achieve an intermediate level of luminance ("grey shades" or "half shades").
The time needed to form an image depends on the number of pixels and on the overall time needed for the addressing operations (erasure addressing and/or writing addressing operations) and sustaining operations.
To reduce the time needed to form an image, it is sought to reduce the overall addressing time. To this effect, the known method consists in controlling the pixels by a semi-selective type of addressing (which is generally a command either for the erasure or for the writing of all the pixels of a given row), followed by a selective type of addressing (wherein one or more selected pixels of this row are controlled so as to be carried to the state which is contrary to the state to which they have been taken by the semi-selective addressing). These two addressing phases form an addressing cycle and, at present, it appears to be difficult to reduce the duration of this addressing cycle to less than 20 microseconds.
Moreover, if it is desired to avoid a visually troublesome flicker, the renewal of the images in the case of dynamic images or with a gray tone should be done at least 50 times per second (frame time of less than 20 microseconds), so that it is difficult for the number of rows written per frame to exceed a thousand.
If the image is formed by only 512 rows, for example, and if the image is renewed 50 times per second, it is possible to obtain four gray tones, taking into account the method used to control these gray tones. Or again, with images of only 256 rows, these 256 rows may each be written four times per second. This leads to 14 levels of luminance or gray tones for each pixel, and an image limited to only 128 lines would enable 64 levels of luminance to be obtained whereas, all the same, it would be desirable to obtain, for example, 128 levels of luminance or gray tones for images of 512 rows.
The current state of the art does not enable sufficient increase in the row-by-row addressing speed, either with a view to obtaining a sufficient number of half-tones as explained above or, again, with a view to other results such as, for example, increasing the number of rows that form an image.